The History and Evolution of the Disk theory for Epsilon Aurigae
The History and Evolution of the Disk theory for Epsilon Aurigae
The History and Evolution of the Disk theory for Epsilon Aurigae
A common question about Epsilon Aurigae is why we theorize the companion to the F-star is a disk-like object. In this thread I've extracted sections of text from the history section of my Ph.D. dissertation in an effort to answer several of the most common questions. Because I have yet to read all of the literature on the subject, Dr. Bob Stencel, my thesis adviser and the Citizen Sky lead scientist will add details as needed. We will both be watching this thread and will answer follow-up questions as they arise.
Noticing the Variability
Why do we call it a 175 year old mystery? The earliest mention of the variability of epsilon Aurigae was in 1821 by an amateur astronomer, High Minister Fritsch of Quedlinburg, Germany. His short written notice (Fritsch, 1824) is written in German and his statement, translated by Roger Mansfield, roughly translates as "I saw the star epsilon Aurigae in the she-goat [asterism] of the Charioteer frequently [to be] so dim compared with zeta and eta that it was barely to be recognized. Has [any]one [else] as yet observed this?"
After this initial observation by Fritsch, several other observers (Argelander, Heis, Oudemans, Schoenfeld, Schawb, Plassmann, Sawyer, Porro, Luizet, Frau von Prittwitz, Kopff, and Goetz) obtained data on epsilon Aurigae, but it wasn't until 1903 that the dimming was proven to occur every 27.1 years (Ludendorff, 1903). A few years later, in 1912, Henry Norris Russell published the first analytic methods for binary star analysis, and in 1915 Harlow Shapley applied the method to epsilon Aurigae with distressing results: the companion was as massive as the F supergiant star primary, but undetected in photometric and spectroscopic observations! In modern terms, this means the mass to light ratio is large, similar to the cosmological problem of “dark matter”.
There are several odd things about the epsilon Aurigae eclipse that make it difficult to analyse and distinctly different than other eclipses. A common theme in literature is the complication of the analysis by out-of-eclipse variations of the F-supergiant. These variations have an amplitude of ~ 0.2 mag in V and have yet to be explained. The variations do not have a single dominant period and are seen in all photometric bands. These variations affect photometry, spectroscopy and polarimetry and have greatly complicated analysis. Non-radial pulsations of the F star are a leading explanation but the signal does not have a simple seismic signature suggestive of stable interior structure.
Fortunately, the actual eclipse is greatly in excess of the out-of-eclipse variations at ~0.8 mag in depth in V. The first observational oddity comes directly from photometry in an equation that describes the ratio of the radii, k, of the two components. Typical values for the primary eclipse, gives us a value of k < 0.38. Theoretically, the square of k follows very closely with the fractional loss of light during the constant phase of the eclipse. The 0.82 mag eclipse represents nearly 50% loss in light, but k^2 for eps Aur's eclipse is ~ 14%, meaning either theory is wrong or something unique is happening. The theory is widely applicable and makes good predictions, therefore we are left to consider other possibilities to explain the eclipse.
Spectroscopic observations further compound the problem: they predict an object that is nearly equal in mass to the F-star, but has almost no out-of-eclipse spectroscopic signature. If the companion nearly equal in mass to the F-star, how can it not be equally luminous? Also, spectroscopic observations during eclipse show additional absorption lines appearing that are red shifted before mid-eclipse and blue-shifted thereafter, suggesting rotation of the companion, that disappear shortly after the photometric eclipse has finished.
Explaining the Eclipse
These observational oddities require a different way of thinking. Several authors have attempted to explain the eclipse phenomenon, sometimes using the most exotic objects in the universe. As others have said before me, the history of Epsilon Aurigae is basically the history of modern astrophysics. Below I include a time line of the models for the eps Aur system as well as their strengths and weaknesses.
The earliest attempt was a comment by Ludendorff in which he stated that a “swarm of meteorites” could cause the eclipse.
Kuiper, Struve, Stromgren (1937)
These three astronomers proposed a model that caused eps Aur to be labelled as the largest star in the Universe in astronomical textbooks for years! Their perspective was that the primary component is a large semitransparent infrared star about which the F-star companion rotates. The eclipse is caused by the F-star going behind the IR star. They propose that the flat-bottomed nature is caused by the F-star's light being scattered/refracted by the IR star's atmosphere.
This theory lost grounds for several reasons, but what I think is the most important comes from calculations by Kraft (1954) in which the density of electrons, and therefore the electron pressure was computed from spectroscopic observations. Kraft's electron pressure of 1-3E-4 dyne/cm^2 was three orders of magnitude smaller than the minimum value Stromgren's model requires (1E-1 dyne/cm^2). At this low of a pressure, scattering by electrons cannot account for the eclipse.
Schoenberg and Jung (1938)
Their so-called Yerkes model is not often discussed in literature because it immediately fails to match observations. In their model they propose a companion that is a star that is sufficiently cool for gas to cool into solid particles during the convective process. The particles – once formed – will, however, lack the necessary support to remain aloft in the star's atmosphere and therefore they fall, are re-heated, and break apart. Although solid particles could easily explain the “gray” nature of the eclipse (i.e. affecting almost all wavelengths equally), a nearly spherical shell of material around the companion star will not give rise to the flat-bottomed light curve during totality.
In the middle of his refutation of the Stromgren model, Kopal presents the first ideas of a disk existing in the system that is in remarkable agreement with current observational evidence. He proposes that the companion to the F-star is a flat, semi-transparent ring (disk) with a radius of ~ 6 AU and an opacity of 0.8. He states the ring should be inclined with respect to the plane of the eclipsing body's orbit. He indicates that this ring should be composed of sold dust particles of 10 – 100 um, rather than excited gas to explain the greyness of the eclipse and that the proximity to the F-star makes it unlikely that Hydrogen would be the dominant element in the ring. Instead, he indicates that the ring is probably formed of crystals of water or light hydrocarbons. It appears he considers the ring to have a mass of ~ 1 solar mass, although his choice of language could mean the whole companion object has that mass. Kopal later offers some small modifications to this model in his 1971 paper.
Following the 1955 eclipse, interests were renewed in eps Aur and Huang offered the first analytical model supporting a disk as the companion object in the Epsilon Aurigae system. He proposed a large block-like structure composed of mostly gas, tantamount to a physically and optically thick disk that obscures the F-star during the eclipse. His model agreed qualitatively with the shape of the observed light curve. He also alludes to observational data that would indicate that the system is asymmetric, with residual gas trailing behind the disk. Because his model explained the shape of the light curve it is the dominant model considered by us today. All theories after this point either extend or provide slight modifications to Huang's disk model. Huang later provided revisions to his model in two papers published in 1974 in response to the criticisms by Wilson (see below).
By the time Cameron published his paper, it was largely agreed that the companion object to the F-star was a disk like object, but one large problem remained. Why is the disk there and how can it be stable? Cameron put forth what is probably the most exotic explanation for the disk: a black hole (called a collapsar in his Nature paper). Observations have been undertaken with X-ray and Gamma ray telescopes in order to confirm Cameron's hypotheses, but thus far they have turned up no evidence for the existence of a black hole in the eps Aur system.
Wilson offered improvements to the Huang model (which precipitated Huang's two replies in 1974), by a computer simulation of the resulting light curve from Huang's thick-disk. Wilson's main criticism was that a thick disk produces a flat-bottomed light curve whereas the observed 1955 light curve has a central brightening as well as “lobes” on either side of mid-eclipse. Wilson therefore proposed a physically thin, optically thick disk with a central opening. Huang disagreed with Wilson's interpretations, but eventually indicated that the true solution probably is somewhere between the Wilson and Huang models.
Eggleton and Pringle (1985)
These two authors explored the long-term stability of the disk and offered an explanation for the nearly 1:1 mass ratio of the system. They proposed that inside of the disk lurks a binary system, composed of two nearly equal mass stars making eps Aur one of only a handful of interacting triple-star systems.
In addition, they also claim that there are two competing mass models. The high-mass model predicts the F-star is a supergiant, whereas the low-mass model indicates that the F-star is something akin to the modern AGB star, and the disk is a remnant of a mass overflow. At this moment, the high-mass model is supported by most authors because the spectra of the F-star appears to be that of an F-supergiant; however, the abnormally low 12C/13C is similar to typical low mass stars along the AGB branch. Given the observational evidence, it will be interesting to see how the high- and low-mass model debate evolves during the current eclipse.
One observational oddity that I did not mention above is the alledged mid-eclipse brightening which shows a ~0.2 mag increase in light near the middle of the photometric minimum. Schmidtke considered the possibility that a gravitational lens could explain the mid-eclipse brightening and found that even in the high-mass case (i.e. ~20:16 solar masses) that the angle observed from Earth was insufficient to explain the mid-eclipse brightening.
Kemp obtained a modest amount of polarization data during the 1983 eclipse and presented the results in his 1986 paper. His then Ph.D. student, Gary Henson, later published all of the polarization data in his Ph.D. thesis and analyzed the out-of-eclipse variations. From Henson's work, there is evidence to support the F-star being a non-radial pulsator, thereby explaining the secondary variations in the light curve. Kemp however investigated the in-eclipse variations. From his data he determined that the disk was tilted with respect to the orbit, and that the orbit crosses the F-star slightly above the star's middle.
The latest tweak to the Huang model is one that proposes the disk is not a continuous aggregate of dust, but instead a series of rings with a Cassini-like division to explain the pre-and post-mid-eclipse brightening. As far as I am aware, there have been no refutations of this model, however a potential weakness is that the older eclipse data does not seem to fit this model very well. It is possible that the disk is undergoing rapid evolution, creating larger gaps between concentric rings, but this requires further investigation and detailed modelling of the disk itself to be validated.
With your observational help, we are obtaining the most complete light curves in the history of studies of epsilon Aurigae. Several research papers are in preparation that need that light curve information in order to provide a clear context:
Interferometric Imaging: we've been most fortunate to image the disk transiting the F star and a report on this will be available early in 2010;
Components in the system: thanks to a combination of ultraviolet, optical and infrared data, we can now detail the nature of the stars and disk in this system – a related report will be unveiled early during 2010 as well.
No black hole: one of the long-running suggestions for how to achieve a high mass, low luminosity companion would be a small black hole – but new X-ray observations put very strong constraints on that idea, nearly eliminating it from consideration. Report in preparation, details soon.